Adhesive drag

Once we have recognized that the contaminant molecules introduce an oscillating interaction energy between surfaces, as illustrated in Fig. 8.7, then we see that more complex adhesion effects must follow. For example, time effects must be observed because the contaminant molecules cannot get into position 

The other significant aspect of adhesive drag is its relation to the surface contamination present on the surface. For example, an alkyd paint film was painted on a glass surface, cross-linked, and then peeled off. For comparison, the same experiment was carried out on a glass surface coated with dimethyldichloro silane, as shown in Fig. 8.10. 

These results gave similar behavior to that of silicone on acrylic, with an apparent equilibrium work of adhesion at low speeds, plus a velocity-dependent 

F iguie 8.10. Kcysults for ped adhesion of pa tnl from silicate glass, showing the efibet of silane 

However, one important energy loss which was explained was the effect of the viscoelastic behavior of the polymer. This was studied by varying the crosslink density of the rubber, to alter the loss of elastic energy as the material relaxed. As the viscoelastic loss increased, so did the adhesive hysteresis, as shown in Fig. 8.13. 

---

Higure 8.16, (a) Aniplificalion of adhesive drag curve by the modulus relaxation (b) sudden crack-stopping effect caused by rate of relaxation. 

Figure 8.19. (a) Dwell-time effect (b) adhesive drag and hysteresis. 

When particles make elastic contact with a surface, equilibrium is not attained immediately. Time is required for the contact spot to enlarge, and more time is needed for the contact to separate when a tensile force is imposed. This is adhesive drag. Indeed, equilibrium may never be attained. On making the contact, the spot size has a certain diameter at a given load. When breaking the contact at an identical load, the contact spot is bigger. This is known as adhesive hysteresis, which was observed by Drutowski in 1969. These effects may be studied systematically with smooth elastomer spheres at zero load as shown in Fig. 9.23." ... 

These observations demonstrate that adhesive drag and hysteresis are not caused by the solid nature of the materials, but by intrinsic properties of the interface, moderated by lossy effects within the materials. 

The phenomena of adhesive drag and hysteresis are relevant to the tack of solids. Certain adhesive tapes need to be tacky so as to grab the attaching... 

Figure 9.23. (a) Apparatus for loading a rubber/glass contact for measurement of contact spot with time, (b) Results for sulfur cross-linked natural rubber, showing the difference between make and break, and indicating adhesive drag and hysteresis. 

Fig. 30. Schematic of the energy barrier causing adhesive drag, in separating a joint from state Wi to state Ws.

The problem of measuring adhesion, in general, is that the curves for peeling have a similar shape, with an apparent work of adhesion plus a large kinetic adhesion drag, but we are not sure exactly where the equilibrium is. So it is important to devise experiments to study both making and breaking the joint in order to define the precise equilibrium point. Three typical experiments are shown in Fig. 33 [13]. 

A more complex problem is to understand the kinetics of adhesion when the system departs from equilibrium. Various mechanisms can interpose a barrier between the molecules, to cause delayed adhesion, adhesive drag and hysteresis. Contemplating these mechanisms is a great challenge for the future. 

---